1. Field of the Invention
This invention relates to a choke for TIG welding machines or plasma cutting installations in which clocked current sources are used and high voltage ignition devices are employed, where the choke can be simultaneously used as ignition transmitter for the ignition device.
2. Description of Prior Art
In one embodiment of clocked current sources, preferably in welding technology, a smoothing choke is mostly employed at the output to the welding torch. It is also customary to employ an ignition device for generating high voltage pulses parallel to the output, that is, parallel to the arc gap, which cuts across the air gap due to the high voltage and forms an ionized, conducting channel for the welding current. In another embodiment, the choke itself is used as an ignition transmitter. In this case, it is provided with a second coil which is connected to an ignition device. It is of no consequence for the further consideration of this invention as to which one of the two embodiments is selected.
In addition to smoothing, the choke must also block the short ignition pulses by means of its inductance and prevent such a high inductive resistance to it so that the high voltage is not short-circuited. Therefore, the energy of the ignition pulses cannot be too high because it occasionally happens that a welder comes in contact with them. Although he would receive an electrical shock, he should not be hurt by it. In order to obtain as high a voltage as possible with limited energy, it is therefore necessary to make the high voltage pulses very short. A typical pulse has a voltage value of approximately 8 to 10 kV and a duration of 1 to 2 .mu.s. If a smoothing choke does not have sufficient inductance, it puts too large a load on the ignition device and the generated pulse voltage decreases, for example, to 6 kV. This results in a substantial reduction in the ignition capability of the machine.
Thus, two requirements are made on the choke simultaneously:
1. Its inductance must be improved to a point that, as a smoothing choke, at a given switching frequency, for example, 25 kHz, it causes the desired amount of waviness of the output current. Simultaneously the dynamics of current regulation are set and the ohmic resistance of the choke must be small so that no noticeable ohmic losses occur at the choke. If this requirement is to be met, a structural form in the shape of a bar core choke with a core of transformer plates is suggested.
2. Its inductance must be such that it optimally blocks the ignition pulses of the high voltage ignition device. This requirement favors a structure with a ferrite core, which ferrite core is suitable for the high pulse frequency.
The unfortunate fact is that the two inductance values which satisfy these requirements have very little in common because their values are separated, for example, by a factor of 5.
Therefore a choke is required, which, with high frequencies (for example, 500 kHz) and low current (for example, 10 A), has high inductance, and with middle frequencies (for example, 25 kHz) and high current (for example, 30 to 400 A), has reduced inductance, along with minimal resistance.
So-called saturation chokes, where the core is of such a size that it goes into saturation past a set choke current, are known. In such chokes, the inductance falls maximally as low as the inductance of an air choke, depending on the core material selected.
The state of the art is a smoothing choke having a ferrite core. This ferrite core then must be of such a size that it still has, for example, one-fifth of its initial permeability at the end, even under full load current. A substantial amount of ferrite is necessary to satisfy this requirement. Consequently, this solution is correspondingly expensive.
The alternative is to make the core of such a size that as an air choke it has the value which is required for its functioning as a smoothing choke. Subsequently, by inserting a ferrite core for the range of small currents, the inductance is raised, for example, by a factor of 5, so that it blocks the ignition pulses. With higher currents this ferrite core then sharply goes into saturation. However, with this alternative, the size of the choke is not optimal, because an air choke has a considerably larger structure than a choke with core, resulting in disadvantageous increases in ohmic resistance.
Use of a choke in a TIG welding installation is disclosed in German Patent DE 20 24 276, while its use in a plasma cutting machine is disclosed in German Patent DE 24 35 020 A1.
U.S. Letters Patent No. 4,103,221 discloses putting together the core of a coil from different materials in order to compensate for the effect of the dc magnetization on the ac behavior.